The distribution of drill sites along the southwest African continental margin was chosen to reconstruct the latitudinal evolution of the Benguela Current system in space and through time. The working area in the Northern Cape Basin (NCB) lies directly seaward of the currently most productive coastal upwelling cells in the area. Therefore, the drill sites are targeted to study the variability of the upwelling system and mechanisms of "leakage" of upwelling sediments into the hemipelagic realm and to analyze and date the onset and early stages of upwelling processes at this latitude.
Two drill sites, NCB-1 and NCB-2, had been originally proposed. These sites were derived from Line AM-1 of Austin and Uchupi (1982). Site NCB-1, in shallow water of 180 m depth, was not included in the final drilling plan because of the lack of Neogene sediments and the general problems of drilling in shelf areas.
The area was visited during Sonne Cruise SO 86 (Bleil et al., 1994), when multichannel seismic data complementary to Line AM-1 were collected (Fig. 3). After the survey of the shallow-water site, technical problems caused us to abandon the profile across the deeper site. During Meteor Cruise M34/1 (Bleil et al., 1996), three additional seismic lines totaling 321 km in length were acquired (Fig. 3) to understand the local tectonic and depositional framework and its relationship to the continental shelf. Together with the previously collected data, the area of the proposed site was now sufficiently studied to precisely locate the drill site.
Site 1084 is located in 1992 m water depth on seismic Line Geosciences Bremen (GeoB)/AWI 96-014 (common depth point [CDP] 2067; see Fig. 4) at the downslope rim of a depositional basin. The sedimentary structures are more complex than in the Walvis Basin, but the uppermost sedimentary sequence (between CDPs 1200 and 3700) of 500–800 ms two-way traveltime (TWT) thickness appears mostly undisturbed and unaffected by earlier mass movements.
Although the area is located more than 200 nmi south of the Walvis Basin, the general seismic and depositional patterns appear to be very similar. The upper seismic unit, which was not penetrated, is characterized by numerous high-amplitude reflectors. The thickness generally decreases seaward, except for at the center of the basin.
Upslope from Site 1084 (from CDP 2400 to 3200), a zone of high-amplitude reflectors is identified beneath 300 ms TWT, which also reveals a decrease in seismic frequency. This might be caused by gas charges or intensified dolomitization, and, consequently, the area was avoided for drilling. At ~700 ms TWT, a thin intercalated slump unit can be identified by diffractions and acoustic transparency.
Several locations have disturbed sediment structures beneath the upper unit (CDP 700–1000, 450 ms TWT; CDP 3200–3700, 800 ms TWT; and CDP 0–800, 800 ms TWT), which seem to have no impact on the integrity of the Neogene sediment cover.
Figure 5 shows a 10-km-long seismic section of Line GeoB/AWI 96-014 across Site 1084 at 1992 m water depth. The seismic pattern reflects hemipelagic deposition without major disturbances or faulting. Some indication for slumping was found at 700 ms TWT sub-bottom depth. Seismic amplitudes are generally high for the upper 700 ms TWT. Several reflectors appear diffuse and patchy, indicating either lateral inhomogeneities or interference.
Figure 6 shows a close-up of the seismic section, plotted against TWT, for a 1-km-long interval near the drill site. Seismic reflectors are compared with the sound velocity log (see "Downhole Logging" section, this chapter), which was also used to recalculate logging depth to TWT. Velocity peaks are associated either with dolomite layers or lithified intervals at depth or with intervals of high organic matter content (black layers; see "Lithostratigraphy" section, this chapter). Black layers are not associated with an increase in logging density, and, accordingly, the change in acoustic impedance is small. The interval of velocity anomalies from 240 to 340 ms TWT is not associated with a distinct increase in seismic energy, although between 200 and 390 ms TWT, amplitudes are generally higher than above. Another interval of lower seismic energy from 390 to 550 ms TWT goes parallel to a smooth velocity log. In the middle of lithologic Unit II, ~570 ms TWT (440 mbsf), an increase in the velocity gradient is associated with a higher variability, which may be related to a change from diatomaceous sediments to nannofossil ooze and clay. This directly affects the returned seismic energy, which shows a significant increase.
Most lithologic changes and variations have a stronger impact on the density in shallow sediments, and it cannot be expected to find their expression also in the velocity log. A decrease in logging sound velocity at 710 ms TWT sub-bottom depth might indicate undercompacted sediments. This decrease coincides well with the observation of a zone of low energy and diffraction, which was tentatively identified as a slump interval above and is confirmed by a disturbed unit with cross-bedding, as described in the "Lithostratigraphy" section (this chapter).